1. Field of the Invention
The present invention relates generally to tape drives, which process data on a magnetic tape by passing the tape alongside a tape head. More particularly, the present invention describes an improved servo system for controlling tape head position relative to the magnetic tape, and for detecting, monitoring, and analyzing servo performance as well as servo errors occurring in the servo system.
2. Description of the Related Art
Data processing systems typically require a large amount of data storage. Effective data processing systems efficiently access, modify, and re-store data within their data storage. Data storage is typically provided in several different mediums, each medium characterized by the time to access the data and the cost to store the data. A first type of data storage medium involves electronic memory, usually dynamic or static random access memory (DRAM or SRAM). Electronic memories take the form of semiconductor integrated circuits where millions of bytes of data can be stored on each circuit, with access to such bytes of data measured in nanoseconds. The electronic memory provides the fastest access to data since such access is accomplished at electronic speeds.
A second type of data storage medium involves direct access storage devices (DASD). DASD storage, for example, can comprise magnetic and/or optical disks. The disks are rotatably mounted within a protected environment and data bits are stored as micrometer-sized magnetically or optically altered spots on a disk surface. Each disk is divided into many concentric tracks, or closely spaced circles. The data is stored serially, bit by bit, along each track. Each DASD contains an access mechanism, known as a head disk assembly (HDA), which typically includes one or more read/write heads. The HDA exchanges data with the surface of each disk as the disk rotates relative to a corresponding read/write head. DASDs can store gigabytes of data, with access to such data typically measured in milliseconds (orders of magnitudes slower than electronic memory). Accessing data already stored on DASD is slower because the disk and HDA must be physically advanced to the desired data storage location.
A third type of data storage medium includes tapes, tape libraries, and optical libraries. Access to data is much slower in a library since a robot is needed to select and load the data storage cartridge. An advantage of these storage systems is the reduced cost for very large data storage capabilities, on the order of terabytes of data. For example, gigabytes of data can be stored within an individual magnetic tape cartridge. Tape storage is often used for back-up purposes. That is, data stored on a different storage medium, such as DASD, is reproduced for safe keeping on magnetic tape. Presently, access to data stored on tape and/or in a library requires a time period on the order of seconds.
For many businesses, having a back-up data copy is mandatory since a data loss could be catastrophic to the business. A large volume of back-up data, such as terabytes of data, is generally stored in a tape library. The library accesses data by using a robotic mechanism to select a tape cartridge from an array of storage bins. Once the tape cartridge is loaded into a tape drive within the library, data can be read from or written to the tape cartridge. Increasing the data capacity of a tape reduces the cost of backing up data and improves library efficiency.
In the information storage industry, increases in tape data capacities have been achieved, in part, by increasing tape lengths stored within a cartridge or reel. Increased tape length has been mainly accomplished by producing thinner tape substrates. Additional increases in data capacity are made possible by various data compression techniques, for example, Improved Data Recording Capability (IDRC) by International Business Machines, Company, or Lempel-Ziv data compression by Stac Electronics, Inc. Data compression techniques can increase data density by two to five times over non-compressed data.
Advances in magnetic tape media and tape head technologies have generated further increases in data capacity by increasing the number of tracks on a magnetic tape (by narrowing a width of each track), as well as the number of read/write elements on the tape head. An eighteen-track tape for data storage has been a standard for many years. More recently, an IBM 3490-E magnetic tape subsystem for 12.7 mm (1/2 inch) wide tape employs a head element with thirty-six read/write elements and has a tape capacity of 800 megabytes. Data compression techniques can further increase the tape capacity to two or more gigabytes. The 3490-E magnetic tape subsystem performs bi-directional linear recording, as opposed to helical scan recording. By interleaving sets of head elements, the number of tape rewinds is reduced, and performance is improved accordingly. In an interleaved head, element pairs having a read-element/write-element configuration (when viewed towards the face of the head) alternate with element pairs having an opposite write-element/read-element configuration, each pair of elements being associated with one track on the tape. When the tape travels in a first direction, the element pairs having one configuration access the corresponding tracks (such as the even numbered tracks) in a read-after-write manner. Conversely, when the tape travels in an opposite direction, those element pairs having the other configuration access the other corresponding tracks (such as the odd numbered tracks), also in a read-after-write manner. This read-after-write technique may be used to immediately verify that data has been correctly written to the tape, free from any errors.
To further enhance performance, this interleaved head element configuration can be implemented in storage systems that use tape having a large number of tracks. This combination may be implemented using a tape head having an equally large number of closely spaced elements. The 3490-E tape head is an example of one tape head with a magneto-resistive transducer formed through thin film deposition techniques.
Historically, the number of tracks that could be established on the tape media has been limited by the number of corresponding elements that could be fabricated on a tape head. Subsequently, however, tape heads have been designed with fewer sets of read/write elements than the number of tracks on the tape, with the idea of transversely moving the tape head relative to the tape. Advantageously the read/write elements of these tape heads are more densely distributed than in the past, facilitating greater track density on the tape.
In this arrangement the tracks are divided into groups, each group containing the same number of tracks as there are read/write element pairs in the head. Accessing all the groups requires indexing the head transversely relative to the tape path, such as with a stepper motor or voice coil driven springs, into a number of discrete positions corresponding to the number of groups of tracks. A head having eight read/write pairs, for example, can accommodate a tape having thirty-two tracks if the tracks are divided into four groups of eight tracks each and the head has four index positions. The distance of head travel between index positions is minimized by interleaving the groups. For example, each of the four groups containing eight tracks (consecutively numbered 0-31) results in tracks 0, 4, 8 . . . , and 28 being in the first group and accessible by the head being indexed to position zero. Similarly, tracks 1, 5, 9, . . . , and 29 are in the second group, accessible in index position one; tracks 2, 6, 10, . . . , and 30 are in the third group, accessible in index position two; and tracks 3, 7, 11, . . . , and 31 are in the fourth group, accessible in index position three.
Despite advances in data capacity, still further increases are desired, such as would be possible with a 12.7 mm wide tape having 64 or even 128 tracks. However, even when a head is indexed, there is a practical limit to the ability of a multi-track head to accurately and reliably record data to and read data from a tape having such a large number of very narrow tracks. Problems can be caused by track mis-registrations, such as tape edge variations, environmental thermal expansion and contraction and inaccuracies in the path the tape follows in a drive. Problems may similarly be caused by inaccuracies in the formatting of tracks on the tape itself. Even a minute "wobble" in the tape can result in significant signal degradation, such as crosstalk or dropout, since a 12.7 mm tape with 128 tracks corresponds to a track width of about eighty microns.
Using fixed heads, as in the past, made it easier to deal with track mis-registrations, etc. Now, however, not only are the tracks narrower, but the head is servo-controlled. In servo-controlled tape systems, precise alignment of the tape head is maintained by using servo elements of the tape head to closely follow servo tracks stored on the tape media. In tape servo arrangements, the head must not only appear fixed in terms of a home position, but it must provide excellent accuracy while servoing across track groups. Hence, very accurate head-to-tape path adjustments are required. These adjustments include track registration, penetration, yaw, and azimuth. The servo-controlled head must not only be isolated from external shock and vibrations which could cause resonances, and hence track mis-registration, but also must be immune to resonances caused by its own servo-induced motion.
As mentioned above, advanced tape head designs separate the write element from the read element to allow an immediate read check of the data written to a data track on the magnetic tape. The servo element also follows the write element when detecting minor adjustments in the magnetic tape position relative to the tape head. This spatial separation between the write element and the read and servo elements can compound track mis-registration at certain frequencies of tape movement. Since the tape head processes data on the magnetic tape in both a forward and reverse direction, the spatial separation between the write element and the servo element can cause the write element to be positioned at a greater offset from the nominal track position, effectively squeezing data tracks together and increasing the chance of track mis-registration.
As discussed above, tape head registration and adjustment are important concerns in tape servo following system. In this respect, some tape servo systems generate position error signals (PESs) indicative of the tape head's position relative to the desired servo track. One example is found in the disclosure U.S. patent application Ser. No. 08/600,914, filed on Feb. 16, 1996 in the names of A. Chliwnyj and S. Wills, and assigned to IBM Corporation. The Chliwnyj Application is hereby incorporated in its entirety herein by reference. As taught in the Chliwnyj application, error messages may be generated under certain conditions to avoid reading or writing when the read position is determined to be too far off-track to permit writing. When an error occurs, reading and/or writing of data may be halted. Although this system may satisfy the needs of many applications, certain other applications may require different or additional kinds of error information. For example, the known type servo systems do not provide any means for monitoring servo performance prior to occurrence of an error, or for diagnosing an error that has occurred. Having information about servo performance prior to occurrence of an error might be useful for example in determining how certain errors arise, and thus how to avoid such errors.